The abiding concern of this project is to characterize several naturally occurring slow viral diseases of domestic animals that resemble important, etiologically obscure diseases of man. Clinicopathologic, virologic, and immunologic features of the animal diseases are studied to obtain an understanding of the natural history and pathogenesis of scrapie and progressive pneumonia of sheep and Aleutioan disease and transmissible encephalopathy of mink. Information obtained from such studies should provide not only an understanding of the mechanisms by which viruses cause protracted disease but also investigational techniques that may help to determine whether viruses are causally related to certain subacute or chronic diseases of man.